Powder metallurgical technology is technology for producing products (sintered compacts) by compaction-molding metal-based powders used as low materials with a mold and sintering the resultant green compacts.
Powder metallurgical technology is capable of producing machine parts having complicated shapes with high dimensional precision and is thus capable of significantly decreasing the production costs of the machine parts. Therefore, various machine parts produced by applying the powder metallurgical technology are used in many fields. Further, in recent years, the requirement for miniaturization or weight lightening of machine parts has increased, and various raw material powders for powder metallurgy for producing small and lightweight machine parts having sufficient strength have been investigated. For example, Japanese Unexamined Patent Application Publication No. 1-219101 (Patent Document 1), Japanese Unexamined. Patent Application Publication No. 2-217403 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 3-162502 (Patent Document 3) disclose raw material powders for powder metallurgy produced by adhering an alloying powder to surfaces of a pure iron powder or alloy steel powder with a binder (referred to as “segregation-free treatment”). Such powders mainly composed of iron (referred to as an “iron-based powder” hereinafter) are usually produced by adding an additive powder (e.g., a copper powder, a graphite powder, an iron phosphide powder, a manganese sulfide powder, or the like) and a lubricant (e.g., zinc stearate, aluminum stearate, or the like) and the resultant mixed powders are supplied to production of machine parts.
As the pure iron powder or alloy steel powder used as a raw material of the iron-based powder, there are an atomized iron powder, a reduced iron powder, and the like according to the production methods. Here, a pure iron powder may be referred to as an iron powder, but the term “iron powder” in the classification by production methods is used in a broad sense including an alloy steel powder. Hereinafter, the term “iron powder” represents an iron powder in the broad sense. The alloy steel powder includes steel powders other than prealloys, i.e., a partially alloyed steel powder and a hybrid alloyed steel powder,
However, the iron-based powder, the additive powder, and the lubricant have different characteristics (i.e., the shape, particle size, and the like), and thus flowability of a mixed powder is not uniform. Therefore, the following problems (a) to (c) occur:
(a) The iron-based powder, the additive powder, the lubricant, and the like locally unevenly distribute due to the influence of vibration or dropping during transport of the mixed powder to a storage hopper. The deviation due to differences in flowability cannot be completely prevented even by the segregation-free treatment.
(b) Since relatively large spaces are produced between particles of the mixed powder charged in the hopper, the apparent density of the mixed powder decreases.
(c) The apparent density of the mixed powder depositing in a lower portion of the hopper increases over time (i.e., due to the influence of gravitation), while the mixed powder in an upper portion of the hopper is stored at a low apparent density. Therefore, the apparent density of the mixed powder is nonuniform in the upper and lower portions of the hopper.
It is difficult to mass-produce machine parts having uniform strength using such a mixed powder.
In order to solve the above problems (a) to (c), it is necessary to increase flowability of the mixed powder of the iron-based powder, the additive powder, and the lubricant.
Therefore, an iron-based powder mainly composed of an iron powder having a predetermined range of particle diameters is proposed.
On the other hand, U.S. Pat. No. 3,357,818 (Patent Document 5) discloses, as means for improving flowability of a metallurgical powder, a technique of adding finest grained inorganic compounds, particularly oxide compounds (preferably having a particle diameter of 1 or less), in an amount of about 25% of an organic lubricant. Examples of the inorganic compounds include silic acid, titanium dioxide, zirconium dioxide, silicon carbide, iron oxide (Fe2O3), and the like.
In addition, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-515542 (Patent Document 6) discloses a technique for improving flowability of an iron powder for powder metallurgy by adding 0.005 to 2% by mass of a metal oxide, such as SiO2 of less than 500 nm or the like. Also, this publication introduces, as segregation-free treatment, a wet method using a resin such as cellulose or the like as a binder (a method of adhering a binder in a natural liquid state or a solvent solution state to an iron powder and then removing liquid contents such as a solvent and the like) and describes that a method of dry-mixing the metal oxide after the removal of a liquid content is preferred.